1. Field of the Invention
The present invention generally relates to radar systems and, more particularly, to a novel technique for enhancing two-dimensional (2D) resolution for real-beam radar (RBR) employed by autonomous approach and landing guidance (AALG) systems.
2. Description of the Prior Art
Currently, flight operations for commercial, military, and private pilots in adverse weather, at night or in low visibility conditions at airport facilities with minimal or no ground aids is either not permissible or hampered. Flight operations are not permissible due to lack of a ground-based instrument landing system (ILS). On the other hand, flight operations are hampered by conventional range and azimuth resolution generated by radar sensors and employed in existing autonomous approach and landing guidance (AALG) systems. AALG systems are a combination of raster imaging sensors, head-up displays, flight guidance and procedures embodied in a virtual reality heads up display (HUD) mounted in the aircraft cockpit which provide pilots with enhanced situational awareness in the above described zero ceiling/zero visibility conditions. Accordingly, AALG allows a pilot to maneuver an aircraft related to take off landing, rollout, taxiing and terminal parking in so-called global operations (e.g. all weather and obscured visibility conditions) providing a clear real-time view of the runway and ground.
Existing AALG systems employ millimeter wave radar that offers better range resolution than lower frequency microwave radars, which allows penetration of fog, smoke and other obscurants/obstructions far superior to for example infrared sensors. Millimeter wave (MMW) radars are classified in two broad categories of pulsed and continuous wave (CW) radar as well as more narrowly classified according to the specific variations or modes of operation associated with each type or use. One type of MMW radar is real-beam radar (RBR) (or real aperture radar (RAR)), which generates two-dimensional (2D) images in range and azimuth. However, conventional range and azimuth resolution generated by RBR and employed by AALG systems have several limitations as discussed below.
In conventional RBR, range resolution is achieved by transmitting a wideband radio frequency (RF) signal towards a target area. Then, as known to those skilled in the art, a linear frequency modulation (FM) technique widely used in frequency modulation continuous wave (FMCW) radar is employed. The linear FM technique is employed by FMCW radar where a stable frequency continuous wave radio energy signal is produced and modulated by a modulation signal. Modulation signals such, as triangular signals are predominately used for determining range and velocity. However sine, sawtooth and the like are also possibly used as modulation signals. After the modulation signal gradually varies the energy signal, it then mixes with a signal reflected from potential target(s) in the target area to produce a beat signal. Digital signal processing (DSP) is thereafter utilized for detection operations after the beat signals are passed through an Analog to Digital converter. However, the above-described conventional RBR range resolution techniques have drawbacks. For example, conventional range resolution is limited by the bandwidth of the swept frequency of the FMCW radar as well as the actual processed bandwidth.
On the other hand, conventional RBR achieves azimuth resolution by deploying a narrow beamwidth antenna, which is mechanically scanned within a controlled sector in the azimuth dimension. The advantage of this type-scanning antenna for radar imaging in azimuth is that it does not need complicated azimuth processing as required by synthetic aperture radar (SAR). However, the drawback of RBR is that its azimuth resolution is typically low and limited by the azimuth beamwidth, which is physically determined by the antenna aperture in the azimuth dimension. Hence, in order to improve azimuth resolution in the prior art one skilled in the art must make modifications in the radar front-end hardware to improve the image quality.
Conventional RBR also have several other drawbacks such as lacking clutter suppression and poor temporal processing. In conventional RBR, a windowed Fourier transform (FT) is utilized for range profile generator, which in turn renders a RBR image. Although the windowed FT purports to be a clutter suppression technique it actually lacks clutter suppression ability and thus gives low image quality. Conventional temporal processing also fails to improve the RBR image resolution. Temporal processing averages multiple image frames to reduce some clutter at a level determined by the number of image frames.
Hence, there is a need for a signal processing technique which can enhance both range and azimuth resolution for RBR employed by AALG systems without requiring modification of the radar front-end hardware to improve the image quality. That is it is highly desirable to enhance conventional range and azimuth resolution of RBR while suppressing clutter background so that both visual and automatic feature extraction (e.g., runway edge, road, shadow, etc) and target detection (e.g., vehicle, building, etc) capabilities can be improved for situational awareness (SA) applications such as AALG systems.